qed: convert to .bdrv_co_is_allocated()

qed: convert to .bdrv_co_is_allocated()

The bdrv_qed_is_allocated() function is a synchronous wrapper around
qed_find_cluster(), which performs the cluster lookup.  In order to
convert the synchronous function to a coroutine function we yield
instead of using qemu_aio_wait().  Note that QED's cache is already safe
for parallel requests so no locking is needed.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>